In the field of broadcast communications, electrical filters are required to separate a desired signal from energy in other bands. These bandpass filters are similar to bandpass filters in other fields. However, unlike most other electrical bandpass filters, filters for broadcast communication must be capable of handling a relatively high input power. For example, a signal input to a broadcast communications filter might have an average power between 5 and 100 kilowatts (kW). Many electronic filters do not have the capacity for such large signal powers.
For many years, high power electrical bandpass filtering has included the use of waveguide cavity filters. In particular, the introduction of dual-mode cavities for microwave filters in 1971 made a significant contribution to the art. Dual-mode filters allowed for a reduction in filter size and mass, and could realize more complex filter functions by their ability to easily couple non-adjacent resonators. Later reductions in size and mass were achieved with the introduction of triple and quadruple mode filters.
While dual-mode waveguide cavity filters have been used often for space and satellite communications, they have also been used for terrestrial television broadcast applications. Indeed, for transmitters operating in a common amplification mode (i.e., a mode in which both audio and video signals are being amplified together), dual-mode filters have become predominant because of their low loss and ability to realize complex filter functions. Moreover, dual-mode filters have been favored for the transmission of analog television signals because of their flexibility in realizing wide pass bandwidths to compensate for frequency drift due to RF heating and ambient temperature changes. However, with the advent of digital television, system requirements have changed. The FCC emissions mask for digital television broadcast stations is very restrictive for power radiated into adjacent channels or out-of-band frequencies. These requirements will not be satisfied by filters that have wide passbands that are allowed to drift.
In the past, waveguide cavities have been developed that are adjustable to compensate for thermal expansion. Paul Goud in Cavity Frequency Stabilization with Compound Tuning Mechanisms, Microwave Journal, March 1971 discloses a waveguide cavity that may be adjusted to compensate for thermal expansion. In FIG. 2 of the article, Goud shows a compound tuning mechanism that may be used to change the effective length of the filter cavity. However, this tuning mechanism requires a manual adjustment of a screw device to make the necessary changes. Moreover, the movable surface is based on a two-section choke. This choke must be unconnected to the sides of the filter, so that it may be moved relative to them. As such, the cavity is unsealed, and is prone to leakage and poorer performance than a sealed filter.
More recently, filter design has addressed the need for narrower bandwidth filters by constructing filters from materials with lower thermal expansion coefficients to minimize the effect of heating on the filter dimensions. In particular, the nickel/steel alloy Invar.RTM. (a registered trademark of Imphy, S.A., Paris, France) has been used as a cavity material. Because of its extremely low degree of thermal expansion, the cavities built with Invar.RTM. suffer less of a dimensional change with heating, and therefore maintain a narrower, more stable passband. However, Invar is also very expensive, and consequently drives up the overall cost of the filter.